Support services with JSON encoding (non-proto)

[vforvalerio87]

As per title, currently the SDK only supports services that use proto encoding; get from the daemon whether the service uses proto or json encoding and adjust the client instance accordingly.

[astroseger]

related to #9 . see switch_to_json_payload_encoding from utils_proto.py in snet-cli. ( I mean that this function shoudn't be re-implemented, but it should be separated in common library)